The breeding goal for food production aims, irrespectively of the species, at: improving gross efficiency (amount of product divided by the amount of feed consumed) en reducing the cost price by: 1) improving productivity (higher yield and financial returns, 2) improving feed conversion (less feed per kg of product and less costs) and 3) improving reproduction, health and survival (less replacement animals needed and less costs). Increasing attention to improve welfare and to reduce environmental impact might be additional breeding goal traits.
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A simplified example of the use of special lines is a three-way cross often applied in in pig breeding: first, sows of a line selected for number of piglets is crossed with a boar of a line selected for growth. Second, crossbred sows are subsequently crossed with boars of a line selected for carcass quality. The result is a lot of piglets born with a good growth and carcass quality. Around 1970 in a sheep breeding trial, ewes from the Finnish Landrace breed (a breed with a high litter size) were mated to a ram of the Ile de France breed (a breed that can be bred irrespective of the season). This resulted in crossbred ewes with a high number of lambs due to three lambing’s in two years’ time. The sire of the lambs was a ram of the Texel breed famous for growth and slaughter quality.